The present invention relates in general to a magnetic recording medium and in particular to a magnetic recording medium for a perpendicular magnetic recording system that is mounted on magnetic recording equipment such as external storage devices of computers.
A perpendicular magnetic recording system is drawing attention as an alternative to a conventional longitudinal magnetic recording system for attaining higher recording density.
A crystalline film of CoCr alloy system is now mainly studied as a material for a magnetic recording layer of a perpendicular magnetic recording medium. In order to use in perpendicular magnetic recording, the crystal orientation is controlled such that c-axis of the CoCr alloy system having hcp structure aligns perpendicular to the film surface (c-plane is parallel to the film surface). In order to achieve higher density in the CoCr alloy system in the future, attempts are being made for finer grain size, reduction of dispersion of grain size distribution, and decrease in magnetic interaction between grains.
A method for controlling a magnetic recording layer structure to raise recording density in a longitudinal recording medium has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. H8-255342 and U.S. Pat. No. 5,679,473. A magnetic layer in these references, generally called a granular magnetic layer, has a structure in which magnetic crystalline grains are surrounded by nonmagnetic and nonmetallic substance such as an oxide or a nitride. Because the nonmagnetic and nonmetallic grain boundary phase physically separates the magnetic particles in the granular magnetic film, magnetic interaction between the magnetic particles decreases and formation of a zigzag-shaped magnetic domain wall in a transition region of a recording bit is suppressed. A low noise characteristic is considered to be attained for this reason.
Combining the above techniques together, the granular magnetic layer is proposed to apply to a recording layer of a perpendicular magnetic recording medium. IEEE Trans. Mag., Vol. 36, p 2393 (2000), for example, discloses a perpendicular magnetic recording medium comprising a ruthenium underlayer and a magnetic layer of CoPtCrO alloy having a granular structure. Here, superior magnetic characteristic and electromagnetic conversion characteristic are obtained as a result of the improvement in c-axis alignment by increasing the thickness of the ruthenium layer, which is an underlayer for the granular magnetic layer, to more than about 40 nm.
Japanese Unexamined Patent Application Publication No. 2001-264517 discloses a perpendicular magnetic recording medium comprising a nonmagnetic underlayer comprised of a metal or an alloy having a hexagonal closest packed (hcp) crystal structure and a seed layer comprised of a metal or an alloy having a face centered cubic crystal structure disposed beneath the underlayer. Such a structure provided an excellent perpendicular magnetic recording medium even in the case of a very thin film of the underlayer or a very thin total thickness of the underlayer and the seed layer.
In order to accomplish a perpendicular magnetic recording medium with excellent electromagnetic conversion characteristic using a granular magnetic layer, the oxide such as SiO2 and the cobalt alloy that are contained in the target need to be well separated in the laminated magnetic film. It is also important to suppress the grain size distribution including extraordinary fine or expanded particles, for reducing noises.
Indeed low noise may be achieved by the structure in which the nonmagnetic and nonmetallic grain boundary region physically separates the magnetic grains. Nevertheless, if a nonmagnetic underlayer is deposited using a material with hexagonal closest packed or body centered cubic structure that has a spacing of lattice planes different from that of the crystals in the magnetic layer by a relative proportion over 15%, the grain size of the magnetic grains in the magnetic layer cannot be controlled without difficulty by depositing the magnetic layer on such a nonmagnetic underlayer because of inferior lattice matching between the underlayer and the crystal grains in the magnetic layer. That is, to accomplish further low noise in a magnetic recording medium, it is required to precisely control minute and uniform grain size of the magnetic crystalline grains.
In view of the above, it would be desirable to provide a magnetic recording medium for perpendicular recording system that allows effective suppression of magnetic interaction between the magnetic grains by facilitating control of reduction of grain size and deviation thereof.